Protein-tyrosine kinases play a critical role in the regulation of growth and differentiation in eukaryotes. Tyrosine phosphorylation/ dephosporylation cascades are important in regulating the cell cycle, and in interaction with the extracellular environment. The role of tyrosine kinases in species such as Trypanosoma brucei which diverged early in the eukaryotic lineage has not been examined. We have found that T. brucei, the causative agent of African sleeping sickness and related diseases, possesses multiple proteins which react with an antiphosphotyrosine antiserum and contain phosphotyrosine. A subset of these proteins are detected only in specific stages of the life cycle. Tyrosine phosphorylation in these organisms is not decreased by inhibitors of mammalian tyrosine kinases, suggesting important structural differences between parasite and mammalian tyrosine kinases. We now wish to explore differences between host and parasite tyrosine kinases and study the role of these enzymes and their substrates in the regulation of proliferation and differentiation in T. brucei. We will clone a gene encoding a T. brucei tyrosine kinase, and examine its structure, expression, and the subcellular localization and activity of the corresponding protein. We also will initiate studies aimed at understanding the function of a stage-regulated substrate, p4O, and the correlation of tyrosine phosphorylation with that function. The proposed work will provide us with further understanding of the evolution of the control of proliferation and differentiation, and may lead to the development of specific anti-trypanosomal compounds that interact with the tyrosine kinase/phosphatase system. We believe that our findings in the model system of T. brucei will lay the foundation for studying tyrosine phosphorylation in other protozoan parasites.